When load currents of substantial magnitude are interrupted by interrupting devices, such as circuit breakers or switches, large currents, voltages and arcs are produced across the opening contacts of the interruption device. These phenomena are very undesirable. They require utilization of specially constructed massive interruption devices for accommodating the arc voltages and plasma and also require special contact members that are intended to withstand the resulting contact pitting and wear. Nevertheless, contact wear can occur. The described phenomena also introduces substantial current and voltage transients into the power line and load system and substantially increases the time required to complete interruption. Thus, these conventional arrangements are unsatisfactory for some applications.
Alternative interruption, i.e., switching, arrangements have been disclosed for reducing these undesired phenomena and their effects. Generally, they rely upon limiting the current flow through the separating contacts of the interruption device so as to reduce the currents, voltages and the ionization across the opening contacts. Current flow through the opening contacts is reduced by diverting load current from the interruption device to a parallel, i.e., shunt circuit. The shunt path generally includes a device that is switched, i.e., gated on, to divert current from the interruption device. Some arrangements switch on the device upon development of a predetermined arc voltage across the switch. For example, in U.S. Pat. No. 3,809,959-Pucher, the arc voltage attains a value sufficient to break down a spark gap which initiates current diversion. Since diversion is initiated only after the existence of a substantial arc voltage, such systems can not entirely prevent the undesirable consequences of arcing. Arcing is accompanied by production of a plasma, i.e., ionization. The degree of ionization and thus the time required to quench the arc is a function of the arc voltage and current magnitudes. Thus, interruption should occur without substantial arcing.
Some systems have therefore been proposed for diverting load current prior to the existence of substantial arc voltages. In these, the interrupting device is generally shunted by the main electrodes of a switchable solid state device, such as a bipolar transistor, FET or gate turnoff device. The switchable device is turned on by a control signal applied to its control electrode so that the main electrodes shunt the opening contacts of the interrupting device and divert, i.e., bypass, the load current. In some systems, the control signal is initiated prior to the existence of substantial arc voltage to expedite diversion and interruption. The switchable device is then cut off, e.g., by a change of the control signal. The voltage across the diversion circuit, e.g., the switchable device, increases subsequent to cut off, causing a decreasing current flow through the inherent inductance of the system. Current flow continues for some time, since the diversion circuit must essentially dissipate the energy stored in the system inductance and any energy that is still contributed by the source. In some cases, this energy can be entirely dissipated by the switchable solid state device which conducts until current flow terminates. Frequently, however, this energy is at least partially dissipated by a voltage responsive device. For this purpose, a voltage responsive device, such as a varistor, shunts the interruption device, i.e., the switch. The varistor conducts when the voltage across the diversion circuit reaches a predetermined value until current is reduced to zero. Diversion circuits of this type are, for example, disclosed in the following applications and patents which are in the name of E. K. Howell, the subject applicant, and are assigned to the assignee of the subJect application and are herein incorporated by reference: U.S. patent application Ser. No. 874,965 filed June 16, 1986 (which is a Continuation-In-Part of abandoned U.S. Patent application Ser. No. 610,947 filed May 16, 1984) entitled "Solid State Current Limiting Circuit Interrupter"; U.S. Pat. No. 4,631.621 entitled "Gate Turn Off Control Circuit"; and U.S. patent application Ser. No. 681,478 filed Dec. 14, 1984 entitled "Circuit Interrupter Using Arc Commutation".
However, even such systems may not be entirely satisfactory, particularly when load currents of large magnitude are interrupted. Ideally, the contacts of the interrupting device should be opened without any arcing. Current diversion should thus commence, and preferably be complete, prior to opening of the interrupting device. Load current diversion is a function of the ratio between the apparent resistance across that portion of the load circuit that includes the interruption device to the apparent resistance of the diversion circuit that diverts the load current. The contact resistance between the closed contacts of the interruption device is extremely low. For ideal interruption, the diversion circuit should also have an extremely low apparent resistance, i.e., preferably equivalent to a zero ohm shunt. Such an ideal diversion circuit would therefore have substantially no voltage drop while current is diverted. However, diversion circuits of the type described above include one or more serially connected solid state devices which have a finite forward voltage drop across their main electrodes during conduction. Usually, one of th.ese devices is a gated solid state device which is turned on and off by signals applied to a control electrode. Such solid state devices, if of sufficient power capacity and exhibiting sufficient blocking voltage, have a relatively large forward voltage drop during full conduction, i.e., saturation. Thus, the above described diversion circuits may have voltage drops that substantially exceed the voltage across the closed interruption device. This delays load current diversion and thus fails to provide ideal interruption.
Applicant's U.S. Pat. No. 4,636,907 which is assigned to the assignee of the subject application and is herein incorporated by reference, discloses an arrangement for diverting load current prior to opening of the interruption device. It discloses a controlled impedance circuit in series with the interruption device. Responsive to the interruption signal, the impedance value is stepped up from a low value to produce a sufficient voltage drop to fully divert the load current prior to the opening of the interruption device. When used with the above described diversion circuits, a sufficiently high voltage drop must, however, be produced across the impedance to compensate for the voltage drop across the diversion circuit. This may have some undesirable consequences. For example, the controlled impedance may have to be designed so that load current flow through the controlled impedance produces excessive energy dissipation during normal operation when the interrupting device is closed.
Additional design considerations must also be satisfied for interruption of load currents of large magnitude, particularly if the electric circuit includes substantial inductance. For example, load current diversion must be coordinated so that there is no breakdown of the interruption device (hereinafter also referred to as "switching means") subsequent to its original opening. Also, interruption must occur fast so as to protect against excessive, e.g., short circuit, currents.